Superconductivity is a phenomenon of electric resistance rapidly becoming zero in a substance such as a certain metal, alloy, and compound when cooled to the ultra-low temperature. A superconducting cable, which is manufactured by applying the technology of superconductivity and used in electric transmission, is used as a conductor where electricity transmits through the superconducting line. This superconducting cable is attracting attention for its advantages such as reducing the size of the facilities for electric transmission, and improving the efficiency of electric transmission because the superconducting cable is capable of transmitting high current with smaller cross-sectional area.
It is important to keep cooling and to maintain the ultra-low temperature state upon using the superconducting cable for electric transmission so as not to reduce the efficiency of electric transmission by loss of superconducting ability due to temperature increase caused by heat load by usage or heat transfer from outside. As a technology to cool the superconducting cable, known is a technology of cyclic cooling using coolant such as the liquid nitrogen. This is to cool the superconducting cable with a cooled coolant by cooling the coolant with a refrigerator and transferring the cooled coolant to the superconducting cable with a pump. For instance, Patent Document 1 discloses a technology of cooling a superconducting cable by circulating coolant in a unicursal path including a refrigerator, a superconducting cable, a pump, and the refrigerator in this order.
The superconducting cable is occasionally considered and actually employed for use as an alternate current (AC) superconducting cable by an electric power company, for instance. The AC superconducting cable considered and used by an electric power company and the like is used as an electric instrument for supplying electricity to a major consumer of electricity such as the skyscrapers from the underground channel called the cable tunnel. Thus, the AC superconducting cable needs to be provided with power leads that serve as connection for transmitting electricity from outside. Herein, for the AC superconducting cable for electric transmission, providing a power lead for each of both ends of the AC superconducting cable is sufficient. Also, for the AC superconducting cable for electric transmission, two or more lines of superconducting cables for electric transmission are provided in order to stabilize the power supply.
As a result, for the AC superconducting cable for electric transmission, it is not necessary to separately provide a returning line for the coolant by using one of the lines for the outgoing coolant and the other for the returning coolant in the cooling system of the superconducting cable for electric transmission. Thus, it is possible to circulate the coolant while effectively using its low temperature. Also, even when only one line of AC superconductive cable for electric transmission is provided, since the AC superconducting cable for electric transmission is a three-phase AC cable, it is not necessary to separately provide a returning line for the coolant by using one phase for outgoing and the other two phases for returning, which makes it possible to circulate the coolant while effectively using its low temperature.
On the other hand, the superconducting cable may be used not only as the AC electric transmission cable, but also as the superconductive feeder cable for the electric railway, for instance.
Currently, electricity is supplied to the electric railway via the pantograph and the trolley line. Since the trolley line is such a thin line that has high electric resistance, electricity is supplied to the trolley line from the feeder cable approximately every 300 meters to prevent voltage drop in an electric railway vehicle. Therefore, a thick line is used for the feeder cable to make its electric resistance extremely low so as to prevent voltage drop. However, it is impossible to avoid voltage drop because even the lowest electric resistance is a finite resistance. Thus, the feeder cables are provided approximately every three kilometers. Although the feeder cable can be used not only for direct electricity supply to the electric railway but also for interchange of electricity between substations such as supplying regenerative electric power of an electric railway vehicle to another vehicle that is speeding up, it is incapable of performing two electric transmissions at the same time. Thus, the feeder cable normally takes its role mainly on transmitting electricity to a railway vehicle. Further, as most of the electric railways in Japan use direct current, employing the superconducting cables as the feeder cables causes the electric resistance to become zero, which results in various advantages such as expanding the intervals between the substations, omitting some of the substations, and solving the problem of electric erosion by all electricity returning to the substations via the superconducting cable.